Wireless visible light communication is a next-generation communication technology where communication is performed by using visible light having a wavelength range from 380 nm to 780 nm. This technology can be used as a system for transmitting data by using light of indoor or outdoor illumination, advertizing lighting panels, signal lamps, streetlamps, or the like. This system can be used in hospitals, airplanes, or the like where use of radio wave is limited. In addition, in the light communication technology, communication can be performed through modulation of a visible light emitting apparatus such as an illumination apparatus, and thus, much attention is paid to the technology as a next-generation high-speed transmission technology. In particularly, in the light communication using visible LEDs (light emitting diodes), high speed modulation can be performed, and security-enhanced ubiquitous communication networks can be implemented without a high-cost infrastructure or without harm to human bodies. Currently, standardization of the afore-mentioned light communication technology is in progress in 802.15.7 Task Group of IEEE (Institute of Electrical and Electronics Engineers).
On the other hand, since the LED is a semi-permanent device, once it is installed, there is almost no necessity for replacement or maintenance. In addition, since the LED has very small power consumption, electric energy can be saved. In addition, since the LED can be implemented as a small-sized, light-weighted device, its installation site is not greatly limited. Therefore, the LED can be used in various applications such as indoor/outdoor illumination and car lamps.
Therefore, the market of LED illumination using the LEDs has been rapidly expanded, and the light communication technology using the LEDs is a good prospect for ubiquitous communication infrastructure such as an intelligent traffic system. The theoretical background for transmission of data on LED illumination is that illumination light beam is allowed to blink at a high speed. When the data is carried on the blinking light beam of the LED illumination to be transmitted, since human eyes are insensitive to the blinking of 100 times or more per second, the human being recognizes the LED illumination as simple illumination, and thus, illumination and communication can be simultaneously implemented through the blinking. In the beginning of the technology using the LED illumination as a communication method, an OOK (On-Off Keying) method is mainly used where data are transmitted simply through ON and OFF of the LED illumination. Among other communication methods using the LED illumination, an NRZ-OOK (Non-Return-to-Zero On-Off Keying) method is most useful for increasing the communication rate except for the difficulty in synchronization. In the binary transmission of the NRZ-OOK method, ‘1’ is transmitted through ON pulses, and ‘0’ is transmitted through OFF pulses.
In addition, M-ary Pulse Amplitude Modulation (hereinafter, referred to as an ‘M-PAM’) method where a signal is modulated into M levels to be transmitted can be used for high speed data transmission. The above-described NRZ-OOK method is an M-PAM method with M=2. As M is increased, the high speed data transmission can be implemented. However, in this case, a decrease in communication performance due to noise needs to be considered.
On the other hand, in the case where data are modulated through an NRZ-OOK (Non-Return-to-Zero On-Off Keying) method or an M-PAM method to be transmitted, since some kinds of data are transmitted in a mixture of the ON/OFF pulses or multi-level pulses, the brightness of illumination is always smaller than the brightness in the case where the maximum value is ON. In general, for communication, source-coded data need to have ‘1’ and ‘0’ in the same numbers. Therefore, in the case where the source-coded data are carried on the LED illumination to be transmitted, there is a problem in that the bright of illumination is smaller by 50% than the brightness in the state where the value is always ON. In addition, at first, in the side where the LED illumination is installed, a decrease in brightness due to the visible light communication is expected not to be considered, and a brightness adjusting function of adjusting the maximum value of brightness of the illumination to be in a range from 0 to 100%, that is, a dimming level adjusting function is included as a unique function of the LED illumination. In the LED illumination, the brightness adjusting is implemented by reducing the amplitude of the illumination light, or it is implemented by adjusting an ON/OFF interval ratio like pulse width modulation (hereinafter, referred to as PWM). The PWM is more generally used. On the other hand, in the case where the NRZ-OOK (Non-Return-to-Zero On-Off Keying) method is used for the LED illumination and the communication, the ON/OFF interval ratio is adjusted to be about 50%. Therefore, in the case where light communication is to be performed by using the LED illumination, the dimming function of adjusting the maximum value of the brightness of illumination to be in a range from 0 to 100% and the communication function of adjusting the ON/OFF interval ratio to be 50% is conflicted. In the case where the M-PAM method is used instead of the OOK method, the same conflict occurs between the dimming function and the communication function.
In order to solve this conflict, S. Kaur, W. Liu, and D. Castor proposed “VLC Dimming Proposal” in IEEE 802.15-09-0641-00-0007 on September, 2009. In the “VLC Dimming Proposal”, a time multiplexing method for communication and illumination is used where the ON interval and the OFF interval are forcibly added to adjust the entire dimming levels without a change of the communication interval having a duty ratio of 50%. In the time multiplexing method is used, when the dimming level Pd is smaller than 50%, the OFF interval is added, and when the dimming level Pd is larger than 50%, the ON interval is added. For example, if the communication interval is a half of the entire time and the remaining half time is set as the ON interval, the dimming level becomes 75%, and the data transmission rate is decreased by half.
In the case where the time multiplexing method in the related art is used, if the dimming level is denoted by Pd and the efficiency of communication interval is denoted by E0, and if Pd>50%, the relationship between the dimming level and the efficiency of communication interval is expressed by Equation 1.Eo=2−2Pd  [Equation 1]
When the efficiency of communication interval of the case where the entire time is used for the NRZ-OOK communication is defined to be 1, the efficiency is simply E0. If Pd<50%, the efficiency E0 is calculated as the following Equation 2.Eo=2Pd  [Equation 2]
If the efficiency of communication interval is expressed by using Equations 1 and 2, a characteristic of a change in efficiency of communication interval illustrated in FIG. 1 is obtained. FIG. 1 is a graph illustrating a characteristic of a change in efficiency of communication interval according to a dimming level in the related art. Referring to FIG. 1, it can be seen that there is a problem in that the efficiency is rapidly decreased when the dimming level is deviated from 50%.